BACKGROUND
The reaction of nitrogen and hydrogen to provide ammonia by the "Hall Process" is well known art with the commercial production of ammonia being developed in the early 1900s. Ammonia is produced by the direct reaction of hydrogen gas and nitrogen gas over an iron based catalytic surface. EQU 3H.sub.2 +N.sub.2.revreaction.2NH.sub.3
The synthesis reaction is reversible and the thermodynamic equilibrium does not favor ammonia production. The overall conversion is low, thus ammonia synthesis is characterized by the recycle of the ammonia synthesis feedstock gas through a reactor, and withdrawing the product from the resulting gas between each recycle. Based on Le Chatelier's principle, an increase in pressure favors ammonia production and a higher temperatures increase reaction rates but in the latter case the equilibrium concentration of ammonia in the product gas decreases with increasing temperature. In addition, high reaction temperatures increase the degradation of the catalyst. The space velocity, or the ratio of the flux rate of gas at standard conditions per volume of catalyst, is generally between about 8,000 and 60,000 volumes per volume per hour. The space velocity for the ammonia synthesis reaction is generally not critical.
The iron based ammonia synthesis catalysts are known to be poisoned by carbon oxides and oxygen in any form. The use of expensive copper liquid scrubbing processes were developed for removal of such carbon oxides from the ammonia synthesis feedstock gas. A shift conversion of carbon dioxide to methane has also been developed.
Generally, the commercial synthesis of ammonia consists of three steps. First the ammonia synthesis feedstock gas is prepared. This involves generation of hydrogen gas, the introduction of nitrogen in the stoichiometric synthesis proportion, and the removal of impurities and catalysts poisons. Catalyst poisons are mainly carbon dioxide and carbon monoxide, though sulfur will also poison the catalyst. Historically, the carbon monoxide in the gas is converted to hydrogen and carbon dioxide by reaction with steam over catalyst. Carbon dioxide can be removed by water scrubbing. Then the ammonia synthesis feedstock gas is passed through the ammonia synthesis reactor. The ammonia is removed by scrubbing the exiting gas with water and the unreacted ammonia synthesis feedstock gas is recycled as the last step.
The low conversion and resulting need to recycle ammonia synthesis feedstock gas results in a buildup of inert impurities, primarily argon and methane. Thus a purge gas stream must be withdrawn to prevent buildup of these inert impurities in the recycled gas. This purge gas stream has little value, and is often flared. In order to avoid the economic and environmental cost of flaring, ammonia manufacturers have emphasized the use of high purity ammonia synthesis feedstock gas.
Gasification has been used to generate hydrogen gas and fuel gas (also known as synthesis gas or "syn-gas") from hydrocarbon streams such as coal, petroleum coke, residual oil, and other materials for years. The hydrocarbon is gasified in the presence of oxygen which is usually generated by an air separation plant in which nitrogen is removed from the air to form the purified oxygen. The availability of nitrogen and hydrogen have led to the use of gasification as a feedstock preparation unit for ammonia synthesis. Synthesis gas from gasification has also been used as a fuel to combustion turbines.
The production of synthesis gas from the solid and liquid carbonaceous fuels, especially coal, coke, and liquid hydrocarbon feeds, has been utilized for a considerable period of time and has recently undergone significant improvements due to the increased energy demand and the need for clean utilization of otherwise low value carbonaceous material. Synthesis gas may be produced by heating carbonaceous fuels with reactive gases, such as air or oxygen, often in the presence of steam or water in a gasification reactor to obtain the synthesis gas which is withdrawn from the gasification reactor.
The synthesis gas may be then further treated often by separation to form a purified hydrogen gas stream. The synthesis gas stream can be processed to obtain a hydrogen gas stream of greater than 99.9 mole percent purity. By product nitrogen gas may be taken from the oxygen plant, purified, and then introduced to the hydrogen gas to create the ammonia synthesis feedstock gas.
In spite of these developments, what is needed in the industry is an effective method of utilizing the purge gas stream from the ammonia synthesis reactor so that the tolerances on the purity of the ammonia synthesis feedstock gas purity limitations can be relaxed.